In some cases, multijunction solar cells can exhibit high conversion efficiencies relative to single junction solar cells. However, to achieve highly efficient multijunction solar cells, the use of low bandgap subcells such as 1.0 eV and 0.68 eV subcells can be important. Some multijunction solar cells can exhibit a large lattice mismatch between the substrate and the semiconductor layers of one or more subcells of the multijunction device, especially low bandgap subcells. For this reason a step graded buffer layer is sometimes used to connect subcells having large lattice mismatches. Unfortunately, some existing buffer layers require high thermal loads during manufacture. High thermal loads can degrade the performance of the resulting solar cell in operation.
Therefore, there exists a need for improved semiconductor structures, devices, and manufacturing methods that can provide multijunction solar cells having higher efficiencies than some existing multijunction cells.